1. Field of the Invention
The present invention relates to an earth sensor which is mounted on a satellite or the like and which can stably perform an attitude detection at a high precision.
2. Description of the Related Art
Heretofore, as a system of a sensor for executing an attitude control of a satellite or the like or detecting pointing angles of an observation sensor, there are various systems such as mirror scan type, static thermal irradiation balance type, conical scan type, and mechanical chopping type. Particularly, as for a geosynchronous satellite in which long life and a high precision attitude measurement are demanded, a two-axis earth sensor of a mirror scan type has been solely used in many cases.
FIG. 7 shows a constructional diagram showing an example of a conventional earth sensor of the mirror scan type. In the diagram, reference numeral 1 denotes an infrared photodetector comprising a pair of single elements, namely, a north scanning element for receiving lights which are spot beams used for scanning of the north side of the earth and a south scanning element for receiving lights which are spot beams used for scanning of the south side; 2 an optical package for enclosing the infrared photodetector 1; and 3 a mirror for reflecting lights L which are spot beams used for the scanning, thereby impinging on the photodetector 1.
Reference numeral 4N denotes a preamplifier for a signal of the north scanning element; 4S a preamplifier for a signal of the south scanning element; 5N a comparator which is connected to the preamplifier 4N and which forms a pitch pulse; 5S a comparator which is connected to the preamplifier 4S and which forms a pitch pulse; 6 a mirror scan drive control unit; 7 a drive unit which has a drive coil and an angle encoder and which outputs center pulses; and 8 an angle generator (signal processing unit) for obtaining an angle between the center of the earth and the west end and an angle between the center of the earth and the east end on the basis of pitch pulses P.sub.N and P.sub.S from the comparators 5N and 5S and a center pulse P.sub.c from the drive unit 7.
In the earth sensor, as shown in FIG. 8(a), a spot beam S.sub.N is allowed to scan the north side of the earth E.sub.A from west to east, and a spot beam S.sub.S is allowed to scan the south side of the earth from west to east. The lights L which, are the spot beams S.sub.N, S.sub.S used for the scanning, are received by the single elements of the infrared photodetector 1 and signals C.sub.N and C.sub.S each having a waveform P.sub.Y as shown in FIG. 8(b) are generated. Those signals C.sub.N and C.sub.S are amplified by the preamplifiers 4N and 4S, respectively. The pitch pulses P.sub.N and P.sub.S each having a waveform P.sub..theta. as shown in FIG. 8(c) are formed by the comparators S.sub.N and 5S.
After that, on the basis of the pitch pulses P.sub.N and P.sub.S from the comparators 5N and 5S and the center pulse P.sub.C from the drive unit 7, angles .theta..sub.1W and .theta..sub.2W between the center of the earth and the west end and angles .theta..sub.1E and .theta..sub.2E between the center of the earth and the east end are obtained by the angle generator 8. By using those obtained angles .theta..sub.1W, .theta..sub.2W, .theta..sub.1E, and .theta..sub.2E, a pitch angle .theta..sub.P) and a roll angle .theta..sub.R between the center of the earth and an optical axis of the earth sensor are obtained on the basis of the following expressions. EQU .theta..sub.P =1/4(-.theta..sub.1W -.theta..sub.2W +.theta..sub.1E +.theta..sub.2E) EQU .theta..sub.R =1/2(.theta..sub.1W -.theta..sub.2W +.theta..sub.1E -.theta..sub.2E)
As an example of the earth sensor having a construction other than the above, there is the mirror scan type two-axis earth sensor disclosed in Japanese Patent Application Laid-Open No. 61-200099 (1986). The earth sensor uses a division mirror arranged so as to enable to scan the upper half portion and the lower half portion of the earth and a scan mirror for reciprocatingly scanning the upper half portion and the lower half portion of the earth right and left.
As another example, there is the earth sensor disclosed in Japanese Patent Application Publication Laid-Open No. 61-20480 (1986). The earth sensor is constructed in such a manner that a plurality of infrared detectors for detecting an image of the earth are placed to a periphery of a substrate on which a thermal infrared image of the earth is formed.
As a first problem point of the conventional earth sensor, there is a problem that when the sensor is mounted on the satellite, a mechanical vibration of a satellite body which the satellite has interferes in the mirror drive unit of the earth sensor, so that such an inconvenience that the mirror drive unit causes mechanical resonance occurs.
For example, when frequency components of a wheel, a solar paddle drive unit, the other devices having the mechanical vibration, and the like are resonated with a mirror torsion bar of the earth sensor, such an inconvenience that a normal mirror scan cannot be performed or the like occurs.
As a second problem point, there is a point that in the conventional earth sensor, since its mechanical vibrating mechanism system occupies a large area of the whole structure, a realization of compactness and light-weightedness is close to limits, so that it is difficult to realize to further improve the compactness and the light-weightedness.